More precisely, the present invention relates to the generation, acceleration and propagation of pulsed beams of electrons and plasma, which, when directed against targets made of solid or liquid matter, allow to obtain an explosive expulsion of small amounts of matter, a phenomenon known as ablation. It is believed, without intending to be constrained by any mechanism, that this phenomenon is linked to the release of energy carried by the beam not to the surface of the target but below it, so as to produce the explosion of the portion of material that lies below the surface.
It is known in the background art to produce currents of electrons in vacuum by thermoionic emission or by means of discharges and consequently to accelerate these currents in a corresponding voltage field. The current densities that can be achieved in this manner, however, are not sufficient for some applications. In the background art, electrons generated in vacuum have been deposited through a thin sheet in a chamber containing a gas at low pressure. Although in this manner it is possible to obtain, in this low-pressure area, electron currents with a high current density, the quantity of instruments required is very high and the effect is not satisfactory.
U.S. Pat. No. 4,335,465 discloses a method for generating and accelerating electrons and ions by applying a voltage, in which electrodes are provided which, under the influence of a voltage, supply electrons, and in which a gas at low pressure supplies electrons and ions. The electrodes are spaced one another and are shielded toward the outside. There is at least one gas discharge channel, formed by openings which are provided in each electrode and are aligned along a common axis. A gas which can be ionized at low pressure is provided between the electrodes, and the electrodes are connected at such a voltage as to generate substantially a gas discharge known as “pseudo-spark”. The current density that can be achieved in the low-pressure gas is substantially higher than the density of a current of electrons or ions in vacuum.
Patent DE 3834402 discloses a process in which a magnetically self-focused electron beam or pseudo-spark discharge is received at the anode output of an electrically insulated quartz tube and is carried therein over a certain distance. A slight curvature of the tube does not have an observable effect on beam transport and consequently facilitates the search for the most suitable angle of impact of the beam on the target. To a certain degree, the tube protects the pseudo-spark chamber from ablation vapors and allows differential pumping due to the small transverse cross-section of the pump. The generation of the beam of electrons with the pseudo-spark chamber is technically complicated, since it is also limited as regards beam power and beam divergence.
U.S. Pat. No. 5,576,593 discloses a particle beam accelerator for generating a beam of electrically charged particles. With this accelerator, particles having a preset charge and mass are extracted from a reservoir and are supplied to an acceleration chamber formed between two different electrical potentials, in order to provide a beam to be used in further processes.
In particular, U.S. Pat. No. 5,576,593 discloses an apparatus for accelerating electrically charged particles. The described accelerator comprises a pulsed plasma reservoir of high particle density, a dielectric tubular chamber with an inside diameter d, which runs from said reservoir, at least two mutually spaced electrodes arranged around the tubular chamber, one electrode being arranged along the inside wall of the reservoir, means for evacuating the dielectric tubular chamber in order to retain only a residual gas charge with a pressure p which is low enough so that the product between the pressure p of the gas and the inside diameter d of the dielectric tube (p×d) is low enough to avoid parasitic discharges in the residual gas charge, means for applying a voltage to the electrodes in order to extract charged particles from the reservoir in the dielectric tubular chamber and accelerate the particles inside it so as to form a beam of charged particles in the dielectric tubular chamber, so that the residual gas charge in the dielectric tubular chamber is ionized along its internal wall and polarized, providing repulsive forces on the walls and attractive forces on the axis, which are capable of focusing electrostatically the beam of charged particles that leaves the dielectric tubular chamber.
The above cited phenomenon of ablation can be performed with a commercially available device known as Channel Spark Ablator (CSA), supplied for example by Neocera Inc. This device utilizes the properties of electrical discharges in low-pressure gases. With reference to FIG. 1, which illustrates schematically such a device, the electron beam is generated as follows.
The system shown in FIG. 1 is connected to a vacuum system and is kept at a pressure ranging from 1.5 to 3.5 Pa (1.5 to 3.5×10−2 mbars). A high-voltage DC generator (10-20 kV, 5 mA) is arranged between the hollow cathode (1) and the ground across the bank of capacitors (10-20 nF) (2) and keeps the cathode (1) at a negative voltage with respect to the ground. When the voltage between the cathode and the ground exceeds the discharge value of an air gap device (3), an air spark is induced in said device. This discharge rapidly brings the electrode (13), arranged at the base of the trigger tube (4), to a nil potential. The difference in potential between the trigger electrode (13) and the hollow cathode (1) triggers a discharge in the gas contained in the trigger tube (4), which is focused further by the possible presence of an annular permanent magnet (5). The positive ions of the gas are accelerated toward the base and the walls of the cathode and strike them with enough energy to extract electrons. The expelled electrons feel the acceleration of the electrical field, which propels them to the right in the drawing, and are forced to enter the channel (6) made of insulating material (7), which directs them toward the target (8). Owing to the presence of ionized gas, the charge of the electrons is spatially shielded: the density of the electrons along the axis of the device reaches very high values and the instantaneous current reaches values on the order of 104 A even in the free path portion (9). Due to the dynamics of the discharge, the electrons stripped during the first steps of said discharge are slower than the ones stripped in the final steps, and therefore there is an accumulation effect (the slow ones start earlier and are reached by the fast ones) which leads to the formation of a pulse which has a clearly defined duration (approximately 100 nsec). The electron pulse strikes the target (8), penetrates a few microns below the surface, and releases the energy (approximately 1 J per pulse), giving rise to an ablation of material which is collected on a substrate (10) arranged at an appropriate distance.
Although this device is effective, some problems and limitations are observed, including the fact that part of the useful energy of the capacitors is used to supply the predischarge in the air gap across resistors (11). Moreover, the discharge time is determined by the release of the spark in the air gap, and this depends on several factors which cannot be controlled, such as the microscopic cleanness of the points (12) of the air gap, the composition, pressure and especially the humidity of the ambient air, and cannot be predetermined accurately.
Moreover, the pressure in the vacuum chamber must be kept within an extremely limited range.
Many materials in fact cannot be deposited in the form of a thin film within this pressure range: generally, pressures much lower than 1 Pa (10−2 mbars) are needed.